mmc: break out get_op_cond code to its own function

This code is useful for testing the existance of devices that
do not have card detect capabilities.  This breaks out the core
functionality and leaves the actual init logic and error reporting
in mmc_start_init().

Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefano Babic <sbabic@denx.de>